The present invention relates to equipment for handling slurries, and more specifically to a mixer used for mixing particulate material, such as stucco or the like, with an aqueous slurry, and often combined with other constituents.
For mixing stucco in water, generally rotary mixers are used. A suitable example of a stucco mixer, of the type used producing gypsum-based products, such as in a wallboard production line, is described in U.S. Pat. No. 6,494,609 incorporated by reference. These mixers, employing a rotating impeller to mix the slurry, are dimensionally quite large, which makes the operation inconvenient. A common problem with conventional mixers is that since the velocity of the slurry within the mixer is not uniform, there is a tendency for slow flowing slurry at the periphery of the mixer to prematurely set. This premature setting causes clogging or disruption of the desired high velocity, steady flow of the slurry, in some cases resulting in stopping the whole production line and thereby causes significant loss of revenue.
While mixing, due to the fluid dynamic conditions, the shear is not equally distributed across the slurry. Besides, residence time during mixing is low (˜3 seconds) owing to the demand of the production line. In some cases, this problem is addressed by adding pins to the mixer. However, this has not been an effective solution. Because gypsum slurry is a shear thinning fluid, upon application of higher shear, the viscosity of the slurry decreases. The relationship governing the viscosity (μ), shear rate (γ) is given asμ=kγn-1  (1)where n<1 and k is the flow consistency index. According to this formula, an application of high shear rate can dramatically reduce viscosity. This relationship also illustrates the possibility of the usage of slurry with a low water:stucco ratio by reducing the effective viscosity at a very high shear rate.
Another aspect of conventional stucco technology is that a hopper providing the powdered stucco, gypsum or dry wallboard ingredients should not prematurely contact water prior to entering the mixer. If such premature contact occurs, the tube or passageway used to transmit the dry ingredients to the slurry can become clogged, also interfering with slurry production. Thus, there is a need for an improved slurry mixing apparatus for creating stucco and/or gypsum slurries.